Airborne Closure of Moisture Budget inside Arctic Atmospheric Rivers

Among arctic moist air intrusions, atmospheric rivers (ARs) provide substantial moisture transport over long distances poleward. Along their corridors, warm and moist air masses undergo various transformation processes and can cause regional sea ice decline, especially when they induce precipitation as rain. Quantifying the components of the atmospheric moisture budget in arctic ARs is key to elucidate their precipitation efficiency. We close the AR moisture budget by measurements of the High Altitude LOng range research aircraft (HALO) during the recent HALO-(AC)³ campaign (Spring, 2022) in the vicinity of the Fram Start and Arctic ocean.

Our analysis is based on a strong AR event that HALO observed on two consecutive days during the occurrence of a sequence of moist air intrusions mid of March 2022. Dropsondes detect the vertical atmospheric profile and therefrom quantify the integrated water vapour transport (IVT) along AR cross sections. Applying regression methods then allows calculating the divergence of IVT. Since the limited number of dropsondes may deteriorate such calculations, we estimate the arising uncertainties using the ICOsahedral Nonhydrostatic model (ICON) in a storm-resolving configuration. Retrieved moisture profiles from the microwave radiometer (HAMP) further complement the sporadic sonde-based moisture profiles. We use the nadir cloud and precipitation radar mounted aboard HALO to derive precipitation rates along the flight curtains.

As the comparison with ICON suggests, the set of dropsondes to derive the IVT divergence within a reasonable range. The advection of moisture is roughly twice as strong as mass convergence. Both components act on different heights, with convergence dominating in the boundary layer (0-1 km) near the low-level jet, whereas moisture advection is more elevated (1-4 km). The strongest moisture convergence arises in the warm prefrontal AR sector while precipitation dominates slightly westwards in the AR centre. The investigated AR event caused rain over sea-ice with a melting layer up to 1.5 km. While there was less IVT on the second observation day, mean precipitation increased from the first day. Model simulations show that evaporation makes only a small contribution to the budget.  Within the ICON simulations, the comparison of precipitation purely based on the along-track radar curtain against that over the entire AR corridor indicates that the along-track curtain captures the mean precipitation intensity of the AR corridor, but misrepresents its spatial variability. However, the HALO devices outperform the ICON simulations in terms of the vertical variability of moisture conversion processes.